Magnetic recording medium and magnetic recording and reproducing device

ABSTRACT

The magnetic recording medium including: a non-magnetic support; and a magnetic layer including a ferromagnetic powder and a binding agent on the non-magnetic support, in which a center line average surface roughness Ra measured regarding a surface of the magnetic layer is 1.0 nm to 1.6 nm, and a difference (Safter−Sbefore) between a spacing Safter measured by optical interferometry regarding the surface of the magnetic layer after methyl ethyl ketone cleaning and a spacing Sbefore measured by optical interferometry regarding the surface of the magnetic layer before methyl ethyl ketone cleaning is greater than 0 nm and equal to or smaller than 15.0 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2018-064060 filed on Mar. 29, 2018 and Japanese PatentApplication No. 2019-054750 filed on Mar. 22, 2019. Each of the aboveapplications is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium and amagnetic recording and reproducing device.

2. Description of the Related Art

Magnetic recording media are widely divided into metal thin film typemagnetic recording media and coating type magnetic recording media. Themetal thin film type magnetic recording medium is a magnetic recordingmedium including a magnetic layer of a metal thin film formed by vapordeposition. On the other hand, a coating type magnetic recording medium(for example, see JP2012-043495A) is a magnetic recording mediumincluding a magnetic layer including ferromagnetic powder together witha binding agent. The coating type magnetic recording medium is a usefulmagnetic recording medium as a data storage medium for storing a largecontent of information for a long period of time, because chemicaldurability is more excellent than that of the metal thin film typemagnetic recording medium. Hereinafter, the coating type magneticrecording medium is simply referred to as a magnetic recording medium.

SUMMARY OF THE INVENTION

It is desired to increase a surface smoothness of a magnetic layer in amagnetic recording medium (for example, see paragraph 0003 ofJP2012-043495A). An increase in the surface smoothness of the magneticlayer causes improvement of electromagnetic conversion characteristics.

In recent years, the magnetic recording medium used for data storage isused in a data center in which a temperature and humidity are managed.On the other hand, in the data center, power saving is necessary forreducing the cost. For realizing the power saving, the managingconditions of the temperature and humidity of the data center can bealleviated compared to the current state, or the managing may not benecessary. However, in a case where the managing conditions of thetemperature and humidity are alleviated or the managing is notperformed, the magnetic recording medium is assumed to be exposed to theenvironmental change caused by the weather change or the seasonalchange.

In regards to this point, from the studies of the inventors, it wasclear that, in a magnetic recording medium having a high surfacesmoothness of a magnetic layer, in a case where a temperature change(for example, temperature change of approximately 15° C. to 50° C.)occurs from a low temperature (for example, higher than 0° C. and equalto or lower than 15° C.) to a high temperature (for example, 30° C. to50° C.) under high humidity (for example, in the environment of relativehumidity of approximately 70% to 100%), a phenomenon of a deteriorationin electromagnetic conversion characteristics occurs.

Therefore, an object of an aspect of the invention is to prevent adeterioration in electromagnetic conversion characteristics due to atemperature change from a low temperature to a high temperature underhigh humidity, in a magnetic recording medium having a high surfacesmoothness of a magnetic layer.

According to one aspect of the invention, there is provided a magneticrecording medium comprising: a non-magnetic support; and a magneticlayer including a ferromagnetic powder and a binding agent on thenon-magnetic support, in which a center line average surface roughnessRa measured regarding a surface of the magnetic layer (hereinafter, alsoreferred to as a “magnetic layer surface roughness Ra”) is 1.0 nm to 1.6nm, and a difference (S_(after)−S_(before)) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after methyl ethyl ketone cleaning and a spacing S_(before)measured by optical interferometry regarding the surface of the magneticlayer before methyl ethyl ketone cleaning (hereinafter, also referred toas a “spacing difference (S_(after)−S_(before)) before and after methylethyl ketone cleaning” or simply “difference (S_(after)−S_(before))^(”))is greater than 0 nm and equal to or smaller than 15.0 nm.

In one aspect, the difference (S_(after) S_(before)) may be 2.0 nm to15.0 nm.

In one aspect, the difference (S_(after)−S_(before)) may be 3.0 nm to12.0 nm.

In one aspect, the magnetic recording medium may further comprise anon-magnetic layer including a non-magnetic powder and a binding agentbetween the non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a backcoating layer including a non-magnetic powder and a binding agent on asurface of the non-magnetic support opposite to a surface provided withthe magnetic layer.

In one aspect, the magnetic recording medium may be a magnetic tape.

According to another aspect of the invention, there is provided amagnetic recording and reproducing device comprising: the magneticrecording medium; and a magnetic head.

According to one aspect of the invention, it is possible to provide amagnetic recording medium which includes a magnetic layer having a highsurface smoothness, and in which a deterioration in electromagneticconversion characteristics due to a temperature change from a lowtemperature to a high temperature under high humidity is prevented, anda magnetic recording and reproducing device including this magneticrecording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Recording Medium

One aspect of the invention relates to a magnetic recording mediumincluding: a non-magnetic support; and a magnetic layer including aferromagnetic powder and a binding agent on the non-magnetic support, inwhich a center line average surface roughness Ra measured regarding asurface of the magnetic layer is 1.0 nm to 1.6 nm, and a difference(S_(after)-S_(before)) between a spacing S_(after) measured by opticalinterferometry regarding the surface of the magnetic layer after methylethyl ketone cleaning and a spacing S_(before) measured by opticalinterferometry regarding the surface of the magnetic layer before methylethyl ketone cleaning is greater than 0 nm and equal to or smaller than15.0 nm.

In the invention and the specification, the “methyl ethyl ketonecleaning” means ultrasonic cleaning (ultrasonic output: 40 kHz)performed for 100 seconds by dipping a test piece cut out from themagnetic recording medium into methyl ethyl ketone (200 g) at a liquidtemperature of 20° C. to 25° C. In a case where the magnetic recordingmedium which is a cleaning target is a magnetic tape, a test piecehaving a length of 5 cm is cut out and subjected to methyl ethyl ketonecleaning. A width of the magnetic tape and a width of the test piece cutout from the magnetic tape is normally ½ inches (0.0127 meters).Regarding a magnetic tape having a width other than the width of ½inches (0.0127 meters), a test piece having a length of 5 cm may be cutout and subjected to methyl ethyl ketone cleaning. In a case where themagnetic recording medium which is a cleaning target is a magnetic disk,a test piece having a size of 5 cm×1.27 cm is cut out and subjected tomethyl ethyl ketone cleaning. The measurement of the spacing after themethyl ethyl ketone cleaning described below is performed, after thetest piece after the methyl ethyl ketone cleaning is left in anenvironment of a temperature of 23° C. and relative humidity of 50% for24 hours.

In the invention and the specification, the “surface of the magneticlayer” of the magnetic recording medium is identical to the surface ofthe magnetic recording medium on the magnetic layer side.

In the invention and the specification, the spacing measured by opticalinterferometry regarding the surface of the magnetic layer of themagnetic recording medium is a value measured by the following method.

In a state where the magnetic recording medium (specifically, the testpiece. The same applies hereinafter) and a transparent plate-shapedmember (for example, glass plate or the like) are overlapped onto eachother so that the surface of the magnetic layer of the magneticrecording medium faces the transparent plate-shaped member, a pressingmember is pressed against the side of the magnetic recording mediumopposite to the magnetic layer side at pressure of 5.05×10⁴ N/m (0.5atm). In this state, the surface of the magnetic layer of the magneticrecording medium is irradiated with light through the transparentplate-shaped member (irradiation region: 150,000 to 200,000 μm²), and aspacing (distance) between the surface of the magnetic layer of themagnetic recording medium and the surface of the transparentplate-shaped member on the magnetic recording medium is acquired basedon intensity (for example, contrast of interference fringe image) ofinterference light generated due to a difference in a light path betweenreflected light from the surface of the magnetic layer of the magneticrecording medium and reflected light from the surface of the transparentplate-shaped member on the magnetic recording medium. The light emittedhere is not particularly limited. In a case where the emitted light islight having an emission wavelength over a comparatively wide wavelengthrange as white light including light having a plurality of wavelengths,a member having a function of selectively cutting light having aspecific wavelength or a wavelength other than wavelengths in a specificwavelength range, such as an interference filter, is disposed betweenthe transparent plate-shaped member and a light reception unit whichreceives reflected light, and light at some wavelengths or in somewavelength ranges of the reflected light is selectively incident to thelight reception unit. In a case where the light emitted is light(so-called monochromatic light) having a single luminescence peak, themember described above may not be used. The wavelength of light incidentto the light reception unit can be set to be 500 to 700 nm, for example.However, the wavelength of light incident to the light reception unit isnot limited to be in the range described above. In addition, thetransparent plate-shaped member may be a member having transparencythrough which emitted light passes, to the extent that the magneticrecording medium is irradiated with light through this member andinterference light is obtained.

The interference fringe image obtained by the measurement of the spacingdescribed above is divided into 300,000 points, a spacing of each point(distance between the surface of the magnetic layer of the magneticrecording medium and the surface of the transparent plate-shaped memberon the magnetic recording medium side) is acquired, this spacing isshown with a histogram, and a mode of this histogram is set as thespacing. The difference (S_(after)−S_(before)) is a value obtained bysubtracting a mode before the methyl ethyl ketone cleaning from a modeafter the methyl ethyl ketone cleaning of the 300,000 points.

Two test pieces from the same magnetic recording medium are cut out, avalue S_(before) of the spacing is obtained without performing themethyl ethyl ketone cleaning with respect to the one test piece, and avalue S_(after) of the spacing is obtained after performing the methylethyl ketone cleaning with respect to the other test piece, and thedifference (S_(after)−S_(before)) may be obtained. Alternatively, thedifference (S_(after)−S_(before)) may be obtained by acquiring values ofthe spacing after performing the methyl ethyl ketone cleaning withrespect to the test piece, with which the value of the spacing beforethe methyl ethyl ketone cleaning is acquired.

The above measurement can be performed by using a commercially availabletape spacing analyzer (TSA) such as Tape Spacing Analyzer manufacturedby Micro Physics, Inc., for example. The spacing measurement of theexamples was performed by using Tape Spacing Analyzer manufactured byMicro Physics, Inc.

In the magnetic recording medium, the center line average surfaceroughness Ra measured regarding the surface of the magnetic layer is 1.0nm to 1.6 nm. That is, the magnetic recording medium is a magneticrecording medium including a magnetic layer having a high surfacesmoothness. In such a magnetic recording medium, by setting thedifference (S_(after)−S_(before)) of the spacings before and after themethyl ethyl ketone cleaning to be greater than 0 nm and equal to orsmaller than 15.0 nm, it is possible to prevent a deterioration inelectromagnetic conversion characteristics due to a temperature changefrom a low temperature to a high temperature under high humidity. Asurmise of the inventors regarding this point is as follows.

The reproducing of information recorded on the magnetic recording mediumis generally performed by bringing the surface of the magnetic layer anda magnetic head (hereinafter, also simply referred to as a “head”) intocontact with each other to slide thereon.

Meanwhile, it is thought that, in a case where a temperature change froma low temperature to a high temperature occurs under high humidity,condensation (attachment of moisture) occurs on the surface of themagnetic layer of the magnetic recording medium. It is surmised that thepresence of moisture causes an increase in a coefficient of frictionduring the sliding between the surface of the magnetic layer and themagnetic head. In the magnetic recording medium having a high smoothnessof the surface of the magnetic layer, the inventors have thought that, atendency that the coefficient of friction during the sliding between thesurface of the magnetic layer and the magnetic head easily increases andthe coefficient of friction further increases due to the presence ofmoisture, may be a reason for the occurrence of a deterioration inelectromagnetic conversion characteristics, in a case where atemperature change from a low temperature to a high temperature occursunder high humidity, in the magnetic recording medium having a highsmoothness of the surface of the magnetic layer. Therefore, it isthought that, a decrease in amount of moisture attached to the surfaceof the magnetic layer, in a case where a temperature change from a lowtemperature to a high temperature occurs under high humidity, allows theprevention of an increase in the coefficient of friction, and as aresult, a deterioration in electromagnetic conversion characteristicscan be prevented.

However, a portion (projection) which mainly comes into contact(so-called real contact) with the head during the sliding between thesurface of the magnetic layer and the head, and a portion (hereinafter,referred to as a “base portion”) having a height lower than that of theportion described above are normally present on the surface of themagnetic layer. The inventors have thought that the spacing describedabove is a value which is an index for a distance between the head andthe base portion during the sliding between the surface of the magneticlayer and the head. However, it is thought that, in a case where somecomponents are present on the surface of the magnetic layer, as theamount of the components interposed between the base portion and thehead increases, the spacing is narrowed. Meanwhile, in a case where thecomponents are removed by the methyl ethyl ketone cleaning, the spacingspreads, and accordingly, the value of the spacing S_(after) after themethyl ethyl ketone cleaning is greater than the value of the spacingS_(before) before the methyl ethyl ketone cleaning. Accordingly, it isthought that the difference (S_(after)−S_(before)) of the spacingsbefore and after the methyl ethyl ketone cleaning can be an index forthe amount of the component interposed between the base portion and thehead.

In regards to this point, the inventors have thought that the presenceof the component removed by the methyl ethyl ketone cleaning on thesurface of the magnetic layer promotes the attachment of moisture to thesurface of the magnetic layer, in a case where a temperature change froma low temperature to a high temperature occurs under high humidity.Accordingly, the inventors have surmised that, in a case where thedifference (S_(after)−S_(before)) of the spacings before and after themethyl ethyl ketone cleaning is decreased, that is, a decrease in theamount of the component contributes to prevention of the attachment ofmoisture, and as a result, this allows the prevention of an increase inthe coefficient of friction. Therefore, the inventors have thought that,in the magnetic recording medium having a high smoothness of the surfaceof the magnetic layer, it is possible to prevent a deterioration inelectromagnetic conversion characteristics due to a temperature changefrom a low temperature to a high temperature under high humidity. Withrespect to this, according to the studies of the inventors, acorrelation is not found between the value of the difference of spacingsbefore and after n-hexane cleaning disclosed in JP2012-043495A, and adeterioration in electromagnetic conversion characteristics of themagnetic recording medium having a high smoothness of the surface of themagnetic layer due to a temperature change from a low temperature to ahigh temperature under high humidity. It is surmised that this isbecause the component cannot be removed or cannot be sufficientlyremoved in the n-hexane cleaning.

Details of the component are not clear. Merely as a surmise, theinventors thought that the component may be a component, a molecularweight of which is greater than that of an organic compound normallyadded to the magnetic layer as an additive. The inventors have surmisedas follows regarding one aspect of this component. In one aspect, themagnetic layer is formed by applying a magnetic layer formingcomposition including a curing agent, in addition to the ferromagneticpowder and the binding agent, onto a non-magnetic support directly orthrough another layer and performing a curing treatment. With the curingtreatment here, a curing reaction (crosslinking reaction) between thebinding agent and the curing agent can be performed. However, it isthought that the binding agent which is not subjected to the curingreaction with the curing agent or the binding agent which isinsufficiently subjected to the curing reaction with the curing agent iseasily separated from the magnetic layer and may be present on thesurface of the magnetic layer. The inventors have surmised that themoisture easily adsorbed to the binding agent (for example, functionalgroup including this binding agent) is a reason of the promotion of theattachment of moisture to the surface of the magnetic layer, in a casewhere a temperature change from a low temperature to a high temperatureoccurs under high humidity.

However, the above description is merely a surmise of the inventors andthe invention is not limited thereto.

Hereinafter, the magnetic recording medium will be further described indetail.

Magnetic Layer

Magnetic Layer Surface Roughness Ra

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic recording medium (magneticlayer surface roughness Ra) is 1.0 nm to 1.6 nm. The magnetic layersurface roughness Ra equal to or smaller than 1.6 nm can contribute toexhibition of excellent electromagnetic conversion characteristics bythe magnetic recording medium. The magnetic layer surface roughness Rais preferably equal to or smaller than 1.5 nm, from a viewpoint offurther improving the electromagnetic conversion characteristics.However, in the magnetic recording medium including a magnetic layerhaving a high surface smoothness, the electromagnetic conversioncharacteristics are deteriorated due to a temperature change from a lowtemperature to a high temperature under high humidity, in a case wherethere is no countermeasure. With respect to this, in the magneticrecording medium in which the spacing difference (S_(after)−S_(before))before and after methyl ethyl ketone cleaning is in the range describedabove, it is possible to prevent the occurrence of a deterioration inelectromagnetic conversion characteristics due to a temperature changefrom a low temperature to a high temperature under high humidity, evenin a case where a magnetic layer having a high surface smoothness isincluded. In addition, in a case where the magnetic layer surfaceroughness Ra is equal to or greater than 1.0 nm, it is possible toprevent a deterioration in electromagnetic conversion characteristicsdue to a temperature change from a low temperature to a high temperatureunder high humidity, by setting the spacing difference(S_(after)−S_(before)) before and after methyl ethyl ketone cleaning tobe in the range. From this viewpoint, the magnetic layer surfaceroughness Ra of the magnetic recording medium is equal to or greaterthan 1.0 nm and preferably equal to or greater than 1.1 nm.

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer in the invention and the specification isa value measured with an atomic force microscope (AFM) in a regionhaving an area of 40 μm×40 μm of the surface of the magnetic layer. Asan example of the measurement conditions, the following measurementconditions can be used. The magnetic layer surface roughness Ra shown inexamples which will be described later is a value obtained by themeasurement under the following measurement conditions.

The measurement is performed regarding the region of 40 μm×40 μm of thearea of the surface of the magnetic layer of the magnetic tape with anAFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tappingmode. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed(probe movement speed) is set as 40 μm/sec, and a resolution is set as512 pixel×512 pixel.

The magnetic layer surface roughness Ra can be controlled by awell-known method. For example, the magnetic layer surface roughness Racan be changed in accordance with the size of various powders includedin the magnetic layer (for example, ferromagnetic powder or non-magneticpowder which may be included randomly) or manufacturing conditions ofthe magnetic recording medium. Thus, by adjusting these, it is possibleto obtain the magnetic recording medium having the magnetic layersurface roughness Ra of 1.0 nm to 1.6 nm.

Spacing Difference (S_(after)−S_(before)) Before and After Methyl EthylKetone Cleaning

The spacing difference (S_(after)−S_(before)) before and after methylethyl ketone cleaning measured by optical interferometry regarding thesurface of the magnetic layer of the magnetic recording medium isgreater than 0 nm and equal to or smaller than 15.0 nm. By setting thespacing difference (S_(after)−S_(before)) to be equal to or smaller than15.0 nm, it is possible to prevent a deterioration in electromagneticconversion characteristics due to a temperature change from a lowtemperature to a high temperature under high humidity, in the magneticrecording medium having a high smoothness of the surface of the magneticlayer. From this viewpoint, the difference (S_(after)−S_(before)) isequal to or smaller than 15.0 nm, preferably equal to or smaller than14.0 nm, more preferably equal to or smaller than 13.0 nm, even morepreferably equal to or smaller than 12.0 nm, still preferably equal toor smaller than 11.0 nm, and still more preferably equal to or smallerthan 10.0 nm. As will be described later in detail, the difference(S_(after)−S_(before)) can be controlled by a surface treatment of themagnetic layer in a manufacturing step of the magnetic recording medium.However, as a result of studies of the inventors, it was determinedthat, as the spacing difference (S_(after)−S_(before)) before and afterthe methyl ethyl ketone cleaning becomes 0 nm, in a case where thesurface treatment of the magnetic layer is performed, it is difficult toprevent a deterioration in electromagnetic conversion characteristicsdue to a temperature change from a low temperature to a high temperatureunder high humidity, in the magnetic recording medium having a highsmoothness of the surface of the magnetic layer. The reason is notclear. Merely as a surmise, the inventors thought that, as the spacingdifference (S_(after)−S_(before)) before and after the methyl ethylketone cleaning becomes 0 nm, in a case where the surface treatment ofthe magnetic layer is performed, the component (for example, lubricant)contributing to the improvement of running stability is excessivelyremoved from the magnetic recording medium. From this viewpoint, thespacing difference (S_(after)−S_(before)) of the magnetic recordingmedium before and after the methyl ethyl ketone cleaning is greater than0 nm, preferably equal to or greater than 1.0 nm, more preferably equalto or greater than 2.0 nm, even more preferably equal to or greater than3.0 nm, still preferably equal to or greater than 4.0 nm.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder known as ferromagnetic powder used in the magneticlayer of various magnetic recording media can be used. It is preferableto use ferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density. From this viewpoint, anaverage particle size of the ferromagnetic powder is preferably equal toor smaller than 50 nm, more preferably equal to or smaller than 45 nm,even more preferably equal to or smaller than 40 nm, still preferablyequal to or smaller than 35 nm, still preferably equal to or smallerthan 30 nm, still more preferably equal to or smaller than 25 nm, andstill even more preferably equal to or smaller than 20 nm. Meanwhile,the average particle size of the ferromagnetic powder is preferablyequal to or greater than 5 nm, more preferably equal to or greater than8 nm, even more preferably equal to or greater than 10 nm, stillpreferably equal to or greater than 15 nm, and still more preferablyequal to or greater than 20 nm, from a viewpoint of stability ofmagnetization.

Hexagonal Ferrite Powder

As a preferred specific example of the ferromagnetic powder, hexagonalferrite powder can be used. For details of the hexagonal ferrite powder,descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A,paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 ofJP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can bereferred to, for example.

In the invention and the specification, the “hexagonal ferrite powder”is to be understood to mean ferromagnetic powder from which a hexagonalferrite type crystal structure can be detected as a main phase by X-raydiffraction analysis. The main phase is to be understood to mean astructure to which the diffraction peak with the highest intensity in anX-ray diffraction spectrum obtained by X-ray diffraction analysis isassigned. For example, when the diffraction peak with the highestintensity in an X-ray diffraction spectrum obtained by X-ray diffractionanalysis is assigned to the hexagonal ferrite type crystal structure, itshall be determined that the hexagonal ferrite type crystal structure isdetected as a main phase. When a single structure is only detected byX-ray diffraction analysis, this detected structure is determined as amain phase. The hexagonal ferrite type crystal structure at leastcontains, as constitutional atoms, an iron atom, a divalent metal atom,and an oxygen atom. A divalent metal atom is a metal atom which canconvert into a divalent cation as an ion thereof, and examples thereofinclude alkaline earth metal atoms, such as a strontium atom, a bariumatom, and a calcium atom, and a lead atom. In the invention and thespecification, the hexagonal strontium ferrite powder is to beunderstood to mean powder in which a main divalent metal atom containedtherein is a strontium atom, and the hexagonal barium ferrite powder isto be understood to mean powder in which a main divalent metal atomcontained therein is a barium atom. The main divalent metal atom is tobe understood to mean a divalent metal atom having the highest contentin terms of atom % among divalent metal atoms contained in this powder.However, the divalent metal atom does not include rare earth atoms. Inthe invention and the specification, the rare earth atoms are selectedfrom the group consisting of a scandium atom (Sc), an yttrium atom (Y),and a lanthanoid atom. The lanthanoid atom is selected from the groupconsisting of a lanthanum atom (La), a cerium atom (Ce), a praseodymiumatom (Pr), a neodymium atom (Nd), a promethium atom (Pm), a samariumatom (Sm), an europium atom (Eu), a gadolinium atom (Gd), a terbium atom(Tb), a dysprosium atom (Dy), a holmium atom (Ho), an erbium atom (Er),a thulium atom (Tm), an ytterbium atom (Yb), and a lutetium atom (Lu).

Hereinafter, the hexagonal strontium ferrite powder which is one aspectof the hexagonal ferrite powder will be described in more detail.

The activation volume of the hexagonal strontium ferrite powder ispreferably 800 to 1,600 nm³. The atomized hexagonal strontium ferritepowder showing the activation volume in the range described above issuitable for manufacturing a magnetic recording medium exhibitingexcellent electromagnetic conversion characteristics. The activationvolume of the hexagonal strontium ferrite powder is preferably equal toor greater than 800 nm³ and can also be, for example equal to or greaterthan 850 nm³. In addition, from a viewpoint of further improvingelectromagnetic conversion characteristics, the activation volume of thehexagonal strontium ferrite powder is more preferably equal to orsmaller than 1,500 nm³, even more preferably equal to or smaller than1,400 nm³, still preferably equal to or smaller than 1,300 nm³, stillmore preferably equal to or smaller than 1,200 nm³, and still even morepreferably equal to or smaller than 1,100 nm³. The same can be appliedto the activation volume of the hexagonal barium ferrite powder.

The “activation volume” is a unit of magnetization reversal and an indexshowing a magnetic magnitude of the particles. Regarding the activationvolume and an anisotropy constant Ku which will be described laterdisclosed in the invention and the specification, magnetic field sweeprates of a coercivity He measurement part at time points of 3 minutesand 30 minutes are measured by using a vibrating sample magnetometer(measurement temperature: 23° C.±1° C.), and the activation volume andthe anisotropy constant Ku are values acquired from the followingrelational expression of Hc and an activation volume V. A unit of theanisotropy constant Ku is 1 erg/cc=1.0×10⁻¹ J/m³.

Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2))}

[In the expression, Ku: anisotropy constant (unit: J/m³), Ms: saturationmagnetization (unit: kA/m), k: Boltzmann's constant, T: absolutetemperature (unit: K), V: activation volume (unit: cm³), A: spinprecession frequency (unit: s⁻¹), and t: magnetic field reversal time(unit: s)]

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Thehexagonal strontium ferrite powder can preferably have Ku equal to orgreater than 1.8×10⁵ J/m³, and more preferably have Ku equal to orgreater than 2.0×10⁵ J/m³. In addition, Ku of the hexagonal strontiumferrite powder can be, for example, equal to or smaller than 2.5×10⁵J/m³. However, the high Ku is preferable, because it means high thermalstability, and thus, Ku is not limited to the exemplified value.

The hexagonal strontium ferrite powder may or may not include rare earthatom. In a case where the hexagonal strontium ferrite powder includesrare earth atom, it preferably includes rare earth atom in a content(bulk content) of 0.5 to 5.0 atom %, with respect to 100 atom % of ironatom is 0.5 to 5.0 atom %. In one aspect, the hexagonal strontiumferrite powder which includes rare earth atom can have a rare earth atomsurface portion uneven distribution. The “rare earth atom surfaceportion uneven distribution” of the invention and the specificationmeans that a rare earth atom content with respect to 100 atom % of ironatom in a solution obtained by partially dissolving the hexagonalstrontium ferrite powder with acid (referred to as a “rare earth atomsurface portion content” or simply as a “surface portion content” forrare earth atom) and a rare earth atom content with respect to 100 atom% of iron atom in a solution obtained by totally dissolving thehexagonal strontium ferrite powder with acid (referred to as a “rareearth atom bulk content” or simply as a “bulk content” for rare earthatom) satisfy a ratio of “rare earth atom surface portion content/rareearth atom bulk content >1.0”. The rare earth atom content of thehexagonal strontium ferrite powder is identical to the bulk content.With respect to this, the partial dissolving using acid is to dissolvethe surface portion of particles configuring the hexagonal strontiumferrite powder, and accordingly, the rare earth atom content in thesolution obtained by the partial dissolving is the rare earth atomcontent in the surface portion of the particles configuring thehexagonal strontium ferrite powder. The rare earth atom surface portioncontent satisfying a ratio of “rare earth atom surface portioncontent/rare earth atom bulk content >1.0” means that the rare earthatoms are unevenly distributed in the surface portion (that is, a largeramount of the rare earth atom is present, compared to that inside), inthe particles configuring the hexagonal strontium ferrite powder. Thesurface portion of the specification and the specification means a partof the region of the particles configuring the hexagonal strontiumferrite powder from the inside from the surface.

In a case where the hexagonal strontium ferrite powder includes rareearth atom, the hexagonal strontium ferrite powder preferably includesrare earth atom having a content (bulk content) of 0.5 to 5.0 atom %with respect to 100 atom % of an iron atom. It is surmised that the rareearth atom having the bulk content in the range described above anduneven distribution of the rare earth atom in the surface portion of theparticles configuring the hexagonal strontium ferrite powder contributeto prevention of a decrease in reproducing output during repeatedreproducing. This is surmised that it is because the anisotropy constantKu can be increased due to the rare earth atom having the bulk contentin the range described above included in the hexagonal strontium ferritepowder and the uneven distribution of the rare earth atom in the surfaceportion of the particles configuring the hexagonal strontium ferritepowder. As the value of the anisotropy constant Ku is high, occurrenceof a phenomenon which is so-called thermal fluctuation can be prevented(that is, thermal stability can be improved). By preventing occurrenceof thermal fluctuation, a decrease in reproducing output during repeatedreproducing can be prevented. This is surmised that, the unevendistribution of the rare earth atom in the surface portion of theparticles of the hexagonal strontium ferrite powder may contribute tostabilization of a spin at an iron (Fe) site in a crystal lattice of thesurface portion, thereby increasing the anisotropy constant Ku.

In addition, it is also surmised that, by using the hexagonal strontiumferrite powder having a rare earth atom surface portion unevendistribution as ferromagnetic powder of the magnetic layer, chipping ofthe surface of the magnetic layer due to sliding with a magnetic headcan be prevented. That is, it is surmised that the hexagonal strontiumferrite powder having a rare earth atom surface portion unevendistribution also contributes to improvement of running durability of amagnetic recording medium. It is surmised that, this is because theuneven distribution of the rare earth atom in the surface of theparticles configuring the hexagonal strontium ferrite powder contributesto an interaction between the surface of the particles and an organicsubstance (for example, binding agent and/or additive) included in themagnetic layer, thereby improving hardness of the magnetic layer.

From a viewpoint of further preventing a decrease in reproducing outputduring repeated running and/or a viewpoint of further improving runningdurability, the rare earth atom content (bulk content) is preferably 0.5to 4.5 atom %, more preferably 1.0 to 4.5 atom %, and even morepreferably 1.5 to 4.5 atom %.

The bulk content is a content obtained by totally dissolving thehexagonal strontium ferrite powder. In the invention and thespecification, the content of the atom is a bulk content obtained bytotally dissolving the hexagonal strontium ferrite powder, unlessotherwise noted. The hexagonal strontium ferrite powder which includesrare earth atom may include only one kind of rare earth atom or mayinclude two or more kinds of rare earth atom, as the rare earth atom. Ina case where two or more kinds of rare earth atom are included, the bulkcontent is obtained from the total of the two or more kinds of rareearth atom. The same also applies to the other components of theinvention and the specification. That is, for a given component, onlyone kind may be used or two or more kinds may be used, unless otherwisenoted. In a case where two or more kinds are used, the content is acontent of the total of the two or more kinds.

In a case where the hexagonal strontium ferrite powder includes rareearth atom, the rare earth atom included therein may be any one or morekinds of the rare earth atom. Examples of the rare earth atom preferablefrom a viewpoint of further preventing a decrease in reproducing outputduring repeated reproducing include a neodymium atom, a samarium atom,an yttrium atom, and a dysprosium atom, a neodymium atom, a samariumatom, an yttrium atom are more preferable, and a neodymium atom is evenmore preferable.

In the hexagonal strontium ferrite powder having a rare earth atomsurface portion uneven distribution, a degree of uneven distribution ofthe rare earth atom is not limited, as long as the rare earth atom isunevenly distributed in the surface portion of the particles configuringthe hexagonal strontium ferrite powder. For example, regarding thehexagonal strontium ferrite powder, a ratio of the surface portioncontent of the rare earth atom obtained by partial dissolving performedunder the dissolving conditions exemplified below and the bulk contentof the rare earth atom obtained by total dissolving performed under thedissolving conditions exemplified below, “surface portion content/bulkcontent” is greater than 1.0 and can be equal to or greater than 1.5.The surface portion content satisfying a ratio of “surface portioncontent/bulk content >1.0” means that the rare earth atoms are unevenlydistributed in the surface portion (that is, a larger amount of the rareearth atoms is present, compared to that inside), in the particlesconfiguring the hexagonal strontium ferrite powder. In addition, theratio of the surface portion content of the rare earth atom obtained bypartial dissolving performed under the dissolving conditions exemplifiedbelow and the bulk content of the rare earth atom obtained by totaldissolving performed under the dissolving conditions exemplified below,“surface portion content/bulk content” can be, for example, equal to orsmaller than 10.0, equal to or smaller than 9.0, equal to or smallerthan 8.0, equal to or smaller than 7.0, equal to or smaller than 6.0,equal to or smaller than 5.0, or equal to or smaller than 4.0. However,the “surface portion content/bulk content” is not limited to theexemplified upper limit or the lower limit, as long as the rare earthatom is unevenly distributed in the surface portion of the particlesconfiguring the hexagonal strontium ferrite powder.

The partial dissolving and the total dissolving of the hexagonalstrontium ferrite powder will be described below. Regarding thehexagonal strontium ferrite powder present as the powder, sample powderfor the partial dissolving and the total dissolving are collected frompowder of the same batch. Meanwhile, regarding the hexagonal strontiumferrite powder included in a magnetic layer of a magnetic recordingmedium, a part of the hexagonal strontium ferrite powder extracted fromthe magnetic layer is subjected to the partial dissolving and the otherpart is subjected to the total dissolving. The extraction of thehexagonal strontium ferrite powder from the magnetic layer can beperformed by a method disclosed in a paragraph 0032 of JP2015-91747A.

The partial dissolving means dissolving performed so that the hexagonalstrontium ferrite powder remaining in the solution can be visuallyconfirmed at the time of the completion of the dissolving. For example,by performing the partial dissolving, a region of the particlesconfiguring the hexagonal strontium ferrite powder which is 10% to 20%by mass with respect to 100% by mass of a total of the particles can bedissolved. On the other hand, the total dissolving means dissolvingperformed until the hexagonal strontium ferrite powder remaining in thesolution is not visually confirmed at the time of the completion of thedissolving.

The partial dissolving and the measurement of the surface portioncontent are, for example, performed by the following method. However,dissolving conditions such as the amount of sample powder and the likedescribed below are merely examples, and dissolving conditions capableof performing the partial dissolving and the total dissolving can berandomly used.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 1 mol/L is held on ahot plate at a set temperature of 70° C. for 1 hour. The obtainedsolution is filtered with a membrane filter having a hole diameter of0.1 μm. The element analysis of the solution obtained as described aboveis performed by an inductively coupled plasma (ICP) analysis device. Bydoing so, the surface portion content of the rare earth atom withrespect to 100 atom % of the iron atom can be obtained. In a case wherea plurality of kinds of rare earth atoms are detected from the elementanalysis, a total content of the entirety of the rare earth atoms is thesurface portion content. The same applies to the measurement of the bulkcontent.

Meanwhile, the total dissolving and the measurement of the bulk contentare, for example, performed by the following method.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 4 mol/L is held on ahot plate at a set temperature of 80° C. for 3 hours. After that, theprocess is performed in the same manner as in the partial dissolving andthe measurement of the surface portion content, and the bulk contentwith respect to 100 atom % of the iron atom can be obtained.

From a viewpoint of increasing reproducing output in a case ofreproducing information recorded on a magnetic recording medium, it isdesirable that the mass magnetization as of ferromagnetic powderincluded in the magnetic recording medium is high. In regards to thispoint, in hexagonal strontium ferrite powder which includes the rareearth atom but does not have the rare earth atom surface portion unevendistribution, as tends to significantly decrease, compared to that inhexagonal strontium ferrite powder not including the rare earth atom.With respect to this, it is surmised that, hexagonal strontium ferritepowder having the rare earth atom surface portion uneven distribution ispreferable for preventing such a significant decrease in as. In oneaspect, as of the hexagonal strontium ferrite powder can be equal to orgreater than 45 A·m²/kg and can also be equal to or greater than 47A·m²/kg. On the other hand, from a viewpoint of noise reduction, as ispreferably equal to or smaller than 80 A·m²/kg and more preferably equalto or smaller than 60 A·m²/kg. σs can be measured by using a knownmeasurement device capable of measuring magnetic properties such as avibrating sample magnetometer. Unless stated otherwise, the massmagnetization as is a value measured at a magnetic field strength of 15kOe. With regard to the unit of σs, 1[kOe]=10⁶/4π[A/m]

With regard to the contents (bulk contents) of the constituting atoms ofthe hexagonal strontium ferrite powder, the content of the strontiumatom in the hexagonal strontium ferrite powder can be, for example, 2.0to 15.0 atom % with respect to 100 atom % of the iron atom. In oneaspect, in the hexagonal strontium ferrite powder, the divalent metalatom included in this powder can be only a strontium atom. In anotheraspect, the hexagonal strontium ferrite powder can also include one ormore kinds of other divalent metal atoms, in addition to the strontiumatom. For example, a barium atom and/or a calcium atom can be included.In a case where the divalent metal atom other than the strontium atom isincluded, a content of a barium atom and a content of a calcium atom inthe hexagonal strontium ferrite powder respectively can be, for example,0.05 to 5.0 atom % with respect to 100 atom % of the iron atom.

As the crystal structure of the hexagonal ferrite, a magnetoplumbitetype (also referred to as an “M type”), a W type, a Y type, and a Z typeare known. The hexagonal strontium ferrite powder may have any crystalstructure. The crystal structure can be confirmed by X-ray diffractionanalysis. In the hexagonal strontium ferrite powder, a single crystalstructure or two or more kinds of crystal structure can be detected bythe X-ray diffraction analysis. For example, in one aspect, in thehexagonal strontium ferrite powder, only the M type crystal structurecan be detected by the X-ray diffraction analysis. For example, the Mtype hexagonal ferrite is represented by a compositional formula ofAFe₁₂O₁₉. Here, A represents a divalent metal atom, in a case where thehexagonal strontium ferrite powder has the M type, A is only a strontiumatom (Sr), or in a case where a plurality of divalent metal atoms areincluded as A, the strontium atom (Sr) occupies the hexagonal strontiumferrite powder with the greatest content based on atom % as describedabove. A content of the divalent metal atom in the hexagonal strontiumferrite powder is generally determined according to the type of thecrystal structure of the hexagonal ferrite and is not particularlylimited. The same applies to a content of an iron atom and a content ofan oxygen atom. The hexagonal strontium ferrite powder at least includesan iron atom, a strontium atom, an oxygen atom, may include a rare earthatom, and may or may not include atoms other than these atoms. As anexample, the hexagonal strontium ferrite powder may include an aluminumatom (Al). A content of the aluminum atom can be, for example, 0.5 to10.0 atom % with respect to 100 atom % of the iron atom. From aviewpoint of further preventing a decrease in reproducing output duringrepeated reproducing, the hexagonal strontium ferrite powder includesthe iron atom, the strontium atom, the oxygen atom, and the rare earthatom, and a content of the atoms other than these atoms is preferablyequal to or smaller than 10.0 atom %, more preferably 0 to 5.0 atom %,and may be 0 atom % with respect to 100 atom % of the iron atom. Thatis, in one aspect, the hexagonal strontium ferrite powder may notinclude atoms other than the iron atom, the strontium atom, the oxygenatom, and the rare earth atom. The content shown with atom % describedabove is obtained by converting the content (unit: % by mass) of eachatom obtained by totally dissolving the hexagonal strontium ferritepowder by using the atomic weight. In addition, in the invention and thespecification, a given atom which is “not included” means that thecontent thereof obtained by performing total dissolving and measurementby using an ICP analysis device is 0% by mass. A detection limit of theICP analysis device is generally equal to or smaller than 0.01 ppm(parts per million) based on mass. The expression “not included” is usedas a meaning including that a given atom is included with the amountsmaller than the detection limit of the ICP analysis device. In oneaspect, the hexagonal strontium ferrite powder does not include abismuth atom (Bi).

Metal Powder

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. For details of theferromagnetic metal powder, descriptions disclosed in paragraphs 0137 to0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A canbe referred to, for example.

ε-Iron Oxide Powder

As a preferred specific example of the ferromagnetic powder, ε-ironoxide powder can also be used. In the invention and the specification,the “ε-iron oxide powder” is to be understood to mean ferromagneticpowder from which an ε-iron oxide type crystal structure can be detectedas a main phase by X-ray diffraction analysis. For example, when thediffraction peak with the highest intensity in an X-ray diffractionspectrum obtained by X-ray diffraction analysis is assigned to theε-iron oxide type crystal structure, it shall be determined that theε-iron oxide type crystal structure is detected as a main phase. As amethod for producing ε-iron oxide powder, a method for producing ε-ironoxide powder from goethite and a reverse micelle method has been known.Both of the above-described production methods has been publicly known.Moreover, J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp.S280-5284 and J. Mater. Chem. C, 2013, 1, pp. 5200-5206 can be referredto about a method for producing ε-iron oxide powder where some of Fe aresubstituted with substitutional atoms such as Ga, Co, Ti, Al, and Rh,for example. The method for producing ε-iron oxide powder which can beused as ferromagnetic powder in a magnetic layer of the magneticrecording medium, however, is not limited to these methods.

The activation volume of the ε-iron oxide powder is preferably 300 to1,500 nm³. The atomized ε-iron oxide powder showing the activationvolume in the range described above is suitable for manufacturing amagnetic recording medium exhibiting excellent electromagneticconversion characteristics. The activation volume of the ε-iron oxidepowder is preferably equal to or greater than 300 nm³ and can also be,for example equal to or greater than 500 nm³. In addition, from aviewpoint of further improving electromagnetic conversioncharacteristics, the activation volume of the ε-iron oxide powder ismore preferably equal to or smaller than 1,400 nm³, even more preferablyequal to or smaller than 1,300 nm³, still preferably equal to or smallerthan 1,200 nm³, and still more preferably equal to or smaller than 1,100nm³.

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Theε-iron oxide powder can preferably have Ku equal to or greater than3.0×10⁴ J/m³, and more preferably have Ku equal to or greater than8.0×10⁴ J/m³. In addition, Ku of the ε-iron oxide powder can be, forexample, equal to or smaller than 3.0×10⁵ J/m³. However, the high Ku ispreferable, because it means high thermal stability, and thus, Ku is notlimited to the exemplified value.

From a viewpoint of increasing reproducing output in a case ofreproducing data recorded on a magnetic recording medium, it isdesirable that the mass magnetization as of ferromagnetic powderincluded in the magnetic recording medium is high. In regards to thispoint, in one aspect, as of the ε-iron oxide powder can be equal to orgreater than 8 A·m²/kg and can also be equal to or greater than 12A·m²/kg. On the other hand, from a viewpoint of noise reduction, σs ofthe ε-iron oxide powder is preferably equal to or smaller than 40A·m²/kg and more preferably equal to or smaller than 35 A·m²/kg.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at an imaging magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification ratio of 500,000 toobtain an image of particles configuring the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of a plurality ofparticles is not limited to an aspect in which particles configuring theaggregate directly come into contact with each other, but also includesan aspect in which a binding agent, an additive, or the like which willbe described later is interposed between the particles. A term,particles may be used for representing the powder.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

The content (filling percentage) of the ferromagnetic powder in themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. The components other than the ferromagnetic powder ofthe magnetic layer are at least a binding agent, and one or more kindsof additives may be randomly included. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement of recording density.

Binding Agent and Curing Agent

The magnetic recording medium is a coating type magnetic recordingmedium and includes a binding agent in the magnetic layer. The bindingagent is one or more kinds of resin. As the binding agent, variousresins generally used as the binding agent of the coating type magneticrecording medium can be used. For example, as the binding agent, a resinselected from a polyurethane resin, a polyester resin, a polyamideresin, a vinyl chloride resin, an acrylic resin obtained bycopolymerizing styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,and a polyvinylalkylal resin such as polyvinyl acetal or polyvinylbutyral can be used alone or a plurality of resins can be mixed witheach other to be used. Among these, a polyurethane resin, an acrylicresin, a cellulose resin, and a vinyl chloride resin are preferable.These resins may be homopolymers or copolymers. These resins can be usedas the binding agent even in the non-magnetic layer and/or a backcoating layer which will be described later.

For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight of the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC) under the followingmeasurement conditions. The weight-average molecular weight of thebinding agent shown in examples which will be described later is a valueobtained by performing polystyrene conversion of a value measured underthe following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent including an activehydrogen-containing group can be used. The “active hydrogen-containinggroup” in the invention and the specification is a functional groupcapable of forming a crosslinked structure by a curing reaction of thisgroup with a curable functional group and desorption of hydrogen atomsincluded in this group. Examples of the active hydrogen-containing groupinclude a hydroxy group, an amino group (preferably a primary aminogroup or a secondary amino group), a mercapto group, and a carboxygroup, a hydroxy group, an amino group, and a mercapto group arepreferable, and a hydroxy group is more preferable. A concentration ofthe active hydrogen-containing group in the binding agent including theactive hydrogen-containing group is preferably 0.10 meq/g to 2.00 meq/g.eq indicates equivalent and is a unit not convertible into SI unit. Inaddition, the concentration of the active hydrogen-containing group canalso be shown with a unit “mgKOH/g”. In one aspect, the concentration ofthe active hydrogen-containing group in the resin including the activehydrogen-containing group is preferably 1 to 20 mgKOH/g.

In one aspect, as the binding agent, a binding agent including an acidicgroup can be used. The “acidic group” of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and salt thereof. Specific examples of the acidicgroup include a sulfonic acid group (—SO₃H), a sulfuric acid group(—OSO₃H), a carboxy group, a phosphoric acid group, and salt thereof.For example, salt of sulfonic acid group (—SO₃H) is represented by—SO₃M, and M represents a group representing an atom (for example,alkali metal atom or the like) which may be cations in water or in anaqueous solvent. The same applies to aspects of salt of various groupsdescribed above. As an example of the binding agent including the acidicgroup, a resin including at least one kind of acidic group selected fromthe group consisting of a sulfonic acid group and salt thereof (forexample, a polyurethane resin or a vinyl chloride resin) can be used.However, the resin included in the magnetic layer is not limited tothese resins. In addition, in the binding agent including the acidicgroup, a content of the acidic group can be, for example, 0.03 to 0.50meq/g. The content of various functional groups such as the acidic groupincluded in the resin can be obtained by a well-known method inaccordance with the kind of the functional group. The amount of thebinding agent used in a magnetic layer forming composition can be, forexample, 1.0 to 30.0 parts by mass with respect to 100.0 parts by massof the ferromagnetic powder.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point is the same as regarding a layerformed by using a composition, in a case where the composition used forforming the other layer includes the curing agent. The preferred curingagent is a thermosetting compound, polyisocyanate is suitable. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to, for example. The amountof the curing agent can be, for example, 0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent in the magneticlayer forming composition, and is preferably 50.0 to 80.0 parts by mass,from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes the ferromagnetic powder and the bindingagent, and may include one or more kinds of additives, if necessary. Asthe additives, the curing agent described above is used as an example.In addition, examples of the additive included in the magnetic layerinclude non-magnetic powder (for example, inorganic powder or carbonblack), a lubricant, a dispersing agent, a dispersing assistant, anantibacterial agent, an antistatic agent, and an antioxidant.

Examples of the lubricant include fatty acid, fatty acid ester, andfatty acid amide, and a magnetic layer can be formed by using one ormore kinds selected from the group consisting of fatty acid, fatty acidester, and fatty acid amide.

Examples of fatty acid include lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid, stearic acid, myristic acid, andpalmitic acid are preferable, and stearic acid is more preferable. Fattyacid may be included in the magnetic layer in a state of salt such asmetal salt.

As fatty acid ester, esters of lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid can be used, for example. Specificexamples thereof include butyl myristate, butyl palmitate, butylstearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitandistearate, sorbitan tristearate, oleyl oleate, isocetyl stearate,isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate,and butoxyethyl stearate.

As fatty acid amide, amide of various fatty acid described above isused, and examples thereof include lauric acid amide, myristic acidamide, palmitic acid amide, and stearic acid amide.

A content of fatty acid in the magnetic layer forming composition is,for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts bymass, and more preferably 1.0 to 7.0 parts by mass with respect to 100.0parts by mass of ferromagnetic powder. A content of fatty acid ester inthe magnetic layer forming composition is, for example, 0.1 to 10.0parts by mass and preferably 1.0 to 7.0 parts by mass with respect to100.0 parts by mass of ferromagnetic powder. A content of fatty acidamide in the magnetic layer forming composition is, for example, 0 to3.0 parts by mass, preferably 0 to 2.0 parts by mass, and morepreferably 0 to 1.0 part by mass with respect to 100.0 parts by mass offerromagnetic powder.

In a case where the magnetic recording medium includes a non-magneticlayer between the non-magnetic support and the magnetic layer, thecontent of fatty acid in a non-magnetic layer forming composition is,for example, 0 to 10.0 parts by mass, preferably 1.0 to 10.0 parts bymass, and more preferably 1.0 to 7.0 parts by mass with respect to 100.0parts by mass of the non-magnetic powder. The content of fatty acidester in the non-magnetic layer forming composition is, for example, 0to 10.0 parts by mass and preferably 0.1 to 8.0 parts by mass withrespect to 100.0 parts by mass of the non-magnetic powder. The contentof fatty acid amide in the non-magnetic layer forming composition is,for example, 0 to 3.0 parts by mass and preferably 0 to 1.0 part by masswith respect to 100.0 parts by mass of the non-magnetic powder.

In the invention and the specification, a given component may be usedalone or used in combination of two or more kinds thereof, unlessotherwise noted. In a case where two or more kinds of given componentsare used, the content is a total content of the two or more kinds ofcomponents.

As the non-magnetic powder used for forming the magnetic layer,non-magnetic powder which can function as an abrasive, non-magneticpowder (for example, non-magnetic colloid particles) which can functionas a projection formation agent which forms projections suitablyprotruded from the surface of the magnetic layer, and the like are used.An average particle size of colloidal silica (silica colloid particles)shown in the examples which will be described later is a value obtainedby a method disclosed in a measurement method of an average particlediameter in a paragraph 0015 of JP2011-048878A. As an example of theadditive which can be used in the magnetic layer including the abrasive,a dispersing agent disclosed in paragraphs 0012 to 0022 ofJP2013-131285A can be used as a dispersing agent for improvingdispersibility of the abrasive. For the dispersing agent for improvingdispersibility of the ferromagnetic powder, a description disclosed inparagraph 0035 of JP2017-016721A can be referred to. In addition, forthe dispersing agent, a description disclosed in paragraphs 0061 and0071 of JP2012-133837A can be referred to. For the additive of themagnetic layer, a description disclosed in paragraphs 0035 to 0077 ofJP2016-051493A can be referred to.

The dispersing agent may be included in the non-magnetic layer. For thedispersing agent which may be included in the non-magnetic layer, adescription disclosed in a paragraph 0061 of JP2012-133837A can bereferred to.

As various additives, a commercially available product can be suitablyselected according to the desired properties or manufactured by awell-known method, and can be used with any amount.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recordingmedium may include a magnetic layer directly on the non-magnetic supportor may include a non-magnetic layer including the non-magnetic powderand the binding agent between the non-magnetic support and the magneticlayer. The non-magnetic powder used in the non-magnetic layer may be apowder of an inorganic substance or a powder of an organic substance. Inaddition, carbon black and the like can be used. Examples of theinorganic substance include metal, metal oxide, metal carbonate, metalsulfate, metal nitride, metal carbide, and metal sulfide. Thesenon-magnetic powder can be purchased as a commercially available productor can be manufactured by a well-known method. For details thereof,descriptions disclosed in paragraphs 0146 to 0150 of JP2011-216149A canbe referred to. For carbon black which can be used in the non-magneticlayer, a description of paragraphs 0040 and 0041 of JP2010-024113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50% to 90% by mass andmore preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, aromatic polyamide subjected tobiaxial stretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heat treatmentmay be performed with respect to these supports in advance.

Back Coating Layer

The magnetic recording medium can also include a back coating layerincluding a non-magnetic powder and a binding agent on a surface of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can berandomly included therein, a well-known technology regarding the backcoating layer can be applied, and a well-known technology regarding thelist of the magnetic layer and/or the non-magnetic layer can also beapplied. For example, for the back coating layer, descriptions disclosedin paragraphs 0018 to 0020 of JP2006-331625A and page 4, line 65, topage 5, line 38, of U.S. Pat. No. 7,029,774B can be referred to.

Various Thicknesses

Regarding the thickness of the non-magnetic support and each layer ofthe magnetic recording medium, the thickness of the non-magnetic supportis, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, and morepreferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to theamount of a saturation magnetization of a magnetic head used, a head gaplength, a recording signal band, and the like, and is, for example, 10nm to 100 nm, and is preferably 20 to 90 nm and more preferably 30 to 70nm, from a viewpoint of realization of high-density recording. Themagnetic layer may be at least one layer, or the magnetic layer can beseparated to two or more layers having magnetic properties, and aconfiguration regarding a well-known multilayered magnetic layer can beapplied. A thickness of the magnetic layer which is separated into twoor more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, equal to orgreater than 50 nm, preferably equal to or greater than 70 nm, and morepreferably equal to or greater than 100 nm. Meanwhile, the thickness ofthe non-magnetic layer is preferably equal to or smaller than 800 nm andmore preferably equal to or smaller than 500 nm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.9 μm and even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers and the non-magnetic support of themagnetic recording medium can be obtained by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is exposed by a well-knownmethod of ion beams or microtome, and the exposed cross section isobserved with a scanning electron microscope. In the cross sectionobservation, various thicknesses can be obtained as the thicknessobtained at any one portion, or as an arithmetical mean of thicknessesobtained at a plurality of portions which are two or more portionsrandomly extracted, for example, two portions. Alternatively, thethickness of each layer may be obtained as a designed thicknesscalculated under the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Compositions for forming the magnetic layer, the non-magnetic layer, andthe back coating layer generally include a solvent, together with thevarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. The amount of solvent in each layerforming composition is not particularly limited, and can be identical tothat in each layer forming composition of a typical coating typemagnetic recording medium. A step of preparing the composition forforming each layer includes at least a kneading step, a dispersing step,and a mixing step provided before and after these steps, if necessary.Each step may be divided into two or more stages. All of raw materialsused in the invention may be added at an initial stage or in a middlestage of each step. In addition, each raw material may be separatelyadded in two or more steps.

In order to prepare each layer forming composition, a well-knowntechnology can be used. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-1101-106338A) and JP1989-079274A (JP-H01-079274A) can be referredto. In addition, in order to disperse each layer forming composition,one or more kinds of dispersion beads selected from the group consistingof glass beads and other dispersion beads can be used as a dispersionmedium. As such dispersion beads, zirconia beads, titania beads, andsteel beads which are dispersion beads having high specific gravity arepreferable. These dispersion beads may be used by optimizing a particlediameter (bead diameter) and a filling percentage of the dispersionbeads. As a disperser, a well-known disperser can be used. Each layerforming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm (for example, filter made ofglass fiber or filter made of polypropylene) can be used, for example.

Coating Step

The magnetic layer can be formed, for example, by directly applying themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition to a side of the non-magnetic support oppositeto a side provided with the magnetic layer (or to be provided with themagnetic layer). For details of the coating for forming each layer, adescription disclosed in a paragraph 0051 of JP2010-024113A can bereferred to.

Other Steps

After the coating step, various processes such as a drying process, analignment process of the magnetic layer, and a surface smoothingtreatment (calender process) can be performed. For various steps, adescription disclosed in paragraphs 0052 to 0057 of JP2010-024113A canbe referred to.

In any stage after the coating step of the magnetic layer formingcomposition, the heat treatment of a coating layer formed by applyingthe magnetic layer forming composition is preferably performed. Thisheat treatment can be performed before and/or after the calenderprocess, for example. The heat treatment can be, for example, performedby placing a support, on which the coating layer formed by applying themagnetic layer forming composition is formed, under heated atmosphere.The heated atmosphere can be an atmosphere at an atmosphere temperatureof 65° C. to 90° C., and more preferably an atmosphere at an atmospheretemperature of 65° C. to 75° C. This atmosphere can be, for example, theatmosphere. The heat treatment under the heated atmosphere can be, forexample, performed for 20 to 50 hours. In one aspect, by performing thisheat treatment, the curing reaction of the curable functional group ofthe curing agent can proceed.

One aspect of Preferable Manufacturing Method

As a preferred manufacturing method of the magnetic recording medium, amanufacturing method including wiping out the surface of the magneticlayer with a wiping material permeated with methyl ethyl ketone(hereinafter, also referred to as a “methyl ethyl ketone wipingtreatment”), preferably after the heat treatment can be used. It isthought that the presence of the component capable of being removed bythis methyl ethyl ketone wiping treatment, on the surface of themagnetic layer, promotes the attachment of moisture to the surface ofthe magnetic layer, in a case where a temperature change from a lowtemperature to a high temperature occurs under high humidity. The methylethyl ketone wiping treatment can be performed by using a wipingmaterial permeated with methyl ethyl ketone, instead of a wipingmaterial used in a dry wiping treatment, based on a dry wiping treatmentgenerally performed in the manufacturing step of the magnetic recordingmedium. For example, in the tape-shaped magnetic recording medium(magnetic tape), the methyl ethyl ketone wiping treatment can beperformed on the surface of the magnetic layer, by causing the magnetictape to run between a sending roller and a winding roller, after orbefore slitting the magnetic tape to have a width accommodated in amagnetic tape cartridge, and pressing a wiping material (for example,cloth (for example, non-woven fabric) or paper (for example, tissuepaper) permeated with methyl ethyl ketone to the surface of the magneticlayer of the magnetic tape during running. A running speed of themagnetic tape during the running and a tension applied in a longitudinaldirection of the surface of the magnetic layer (hereinafter, simplyreferred to as a “tension”) can be identical to treatment conditionsgenerally used in the dry wiping treatment generally performed in themanufacturing step of the magnetic recording medium. For example, arunning speed of the magnetic tape in the methyl ethyl ketone wipingtreatment can be approximately 60 to 600 m/min, and the tension can beapproximately 0.196 to 3.920 N (newton). In addition, the methyl ethylketone wiping treatment can be performed at least once. As describedabove, as the spacing difference (S_(after)−S_(before)) before and afterthe methyl ethyl ketone cleaning becomes 0 nm, in a case where thesurface treatment of the magnetic layer is performed, it is difficult toprevent a deterioration in electromagnetic conversion characteristicsdue to a temperature change from a low temperature to a high temperatureunder high humidity, in the magnetic recording medium having a highsmoothness of the surface of the magnetic layer. Therefore, byconsidering this point, it is preferable to set the treatment conditionsof the methyl ethyl ketone wiping treatment and the number of times ofthe treatment.

The polishing treatment and/or the dry wiping treatment generallyperformed in the manufacturing step of the coating type magneticrecording medium (hereinafter, these are referred to as a “dry surfacetreatment”) can also be performed one or more times on the surface ofthe magnetic layer, before and/or after the methyl ethyl ketone wipingtreatment. According to the dry surface treatment, for example, foreignmaterials which are generated during the manufacturing step such asscraps generate due to slitting, and attached to the surface of themagnetic layer can be removed, for example.

Hereinabove, the description has performed using the tape-shapedmagnetic recording medium (magnetic tape) as an example. Regarding thedisk-shaped magnetic recording medium (magnetic disk), various processescan be performed with reference to the above descriptions.

The magnetic recording medium according to one aspect of the inventiondescribed above can be, for example, a tape-shaped magnetic recordingmedium (magnetic tape). The magnetic tape is normally used to beaccommodated and circulated in a magnetic tape cartridge. The recordingand reproducing of information on the magnetic tape can be performed bymounting the magnetic tape cartridge on the magnetic recording andreproducing device and causing the magnetic tape to run in the magneticrecording and reproducing device to cause a contact between the surfaceof the magnetic tape (surface of magnetic layer) and magnetic head toslide thereon. However, the magnetic recording medium according to oneaspect of the invention is not limited to the magnetic tape. As variousmagnetic recording media (magnetic tape, disk-shaped magnetic recordingmedium (magnetic disk) and the like) used in a sliding type magneticrecording and reproducing device, the magnetic recording mediumaccording to one aspect of the invention is suitable. The sliding typedevice is a device in which the surface of the magnetic layer and thehead are in contact with each other and slide, in a case of performingrecording of information on the magnetic recording medium and/orreproducing of the recorded information.

In the magnetic recording medium thus prepared, a servo pattern may beformed by a known method, in order to allow control of tracking of amagnetic head and control of the running speed of the magnetic recordingmedium to be performed in the magnetic recording and reproducing device.The “formation of a servo pattern” can also be referred to as “recordingof a servo signal”. The magnetic recording medium can be a tape-shapedmagnetic recording medium (magnetic tape), and can be a disk-shapedmagnetic recording medium (magnetic disk). Formation of the servopattern in a magnetic tape will be described below, as an example.

The servo pattern is generally recorded along the longitudinal directionof the magnetic tape. Examples of control (servo control) systemsutilizing servo signals include timing-based servo (TBS), amplitudeservo, and frequency servo.

As shown in European Computer Manufacturers Association (ECMA)-319, atiming-based servo technique has been employed in a magnetic tape(generally referred to as “LTO tape”) in accordance with LinearTape-Open (LTO) specifications. In this timing-based servo technique,the servo patterns are configured of consecutive alignment of aplurality of pairs of magnetic stripes (also referred to as “servostripes”), in each pair of which magnetic stripes are not parallel witheach other, in the longitudinal direction of the magnetic tape. Thereason why the servo signal is configured of pairs of magnetic stripes,in each pair of which magnetic stripes are not parallel with each other,is to teach a passing position to a servo signal reading element passingover the servo pattern. Specifically, the pairs of magnetic stripes areformed so that the intervals consecutively change along the widthdirection of the magnetic tape, and relative positions of the servopattern and the servo signal reading element can be determined byreading the intervals with the servo signal reading element. Theinformation on this relative positions enable the data track to betracked. Thus, a plurality of servo tracks are generally set over theservo signal along the width direction of the magnetic tape.

The servo band is configured of servo signals continuously aligned inthe longitudinal direction of the magnetic tape. A plurality of theservo bands are generally provided in the magnetic tape. For example, inan LTO tape, the number of servo bands is five. A region sandwichedbetween the adjacent two servo bands is referred to as a data band. Thedata band is configured of a plurality of data tracks, and data trackscorresponds to respective servo tracks.

In one aspect, information on the number of servo bands (also referredto as information on a “servo band identification (ID)” or a “uniquedata band identification method (UDIM)”) is embedded in each servo bandas shown in Japanese Patent Application Publication No. 2004-318983.This servo band ID is recorded shiftedly such that the position of aspecific pair of servo stripes, among a plurality of servo stripespresent in a servo band, should shift in the longitudinal direction ofthe magnetic tape. Specifically, the degree of shifting the specificpair of servo stripes among the plurality of pairs of servo stripes ischanged by each servo band. Accordingly, the recorded servo band ID isunique by each servo band, and the servo band is uniquely specified byreading one servo band with the servo signal reading element.

As another method for uniquely specifying a servo band, a method using astaggered technique as shown in ECMA-319 can be applied. In thisstaggered technique, a group of a plurality of pairs of magnetic stripes(servo stripes), in each pair of which magnetic stripes are not parallelwith each other and which are placed consecutively in the longitudinaldirection of the magnetic tape, are shiftedly recorded by each servoband in the longitudinal direction of the magnetic tape. A combinationof ways of shifting for each adjacent servo bands is unique in theentire magnetic tape. Accordingly, when a servo pattern is read with twoservo signal reading elements, the servo band can be uniquely specified.

Information indicating a position in the longitudinal direction of themagnetic tape (also referred to as “longitudinal position (LPOS)information”) is also generally embedded in each servo band as shown inECMA-319. This LPOS information is also recorded by shifting theposition of the pair of servo stripes in the longitudinal direction ofthe magnetic tape. Unlike the UDIM information, the same signal isrecorded in each servo band in the case of LPOS information.

Other information different from UDIM information and LPOS informationas mentioned above can also be embedded in the servo band. In this case,the information to be embedded may be different by each servo band likethe UDIM information or may be the same by each servo band like the LPOSinformation.

As a method for embedding information in a servo band, a method otherthan the above-described method may also be employed. For example, amonga group of pairs of servo stripes, a predetermined pair of servo stripesis thinned out to record a predetermined code.

A head for forming a servo pattern is referred to as a servo write head.The servo write head has the same number of pairs of gaps correspondingto the respective pairs of magnetic stripes as the number of servobands. Generally, a core and a coil are connected to each pair of gaps,and a magnetic field generated in the core by suppling a current pulseto the coil can generate a leakage magnetic field to the pair of gaps.When a servo pattern is formed, a magnetic pattern corresponding to apair of gaps can be transferred to the magnetic tape by inputting acurrent pulse while causing a magnetic tape to run over the servo writehead, to form a servo pattern. Thus, the servo pattern can be formed.The width of each gap can be set as appropriate according to the densityof the servo pattern to be formed. The width of each gap can be set to,for example, 1 μm or less, 1 to 10 μm, or 10 μm or larger.

Before forming a servo pattern on the magnetic tape, the magnetic tapeis generally subjected to a demagnetization (erasing) treatment. Thiserasing treatment may be performed by adding a uniform magnetic field tothe magnetic tape using a direct current magnet or an alternate currentmagnet. The erasing treatment includes direct current (DC) erasing andan alternating current (AC) erasing. The AC erasing is performed bygradually reducing the intensity of the magnetic field while invertingthe direction of the magnetic field applied to the magnetic tape. Incontrast, the DC erasing is performed by adding a one-direction magneticfield to the magnetic tape. The DC erasing further includes two methods.The first method is horizontal DC erasing of applying a one-directionmagnetic field along the longitudinal direction of the magnetic field.The second method is a vertical DC erasing of applying a one-directionmagnetic field along the thickness direction of the magnetic tape. Theerasing treatment may be applied to the entire magnetic tape of themagnetic tape, or may be applied to each servo band of the magnetictape.

The direction of the magnetic field of the servo pattern to be formed isdetermined according to the direction of the erasing. For example, whenthe magnetic tape has been subjected to the horizontal DC erasing, theservo pattern is formed so that the direction of the magnetic fieldbecomes reverse to the direction of the erasing. Accordingly, the outputof the servo signal, which can be yielded by reading the servo pattern,can be increased. As shown in Japanese Patent Application PublicationNo. 2012-53940, when a magnetic pattern is transferred to the magnetictape which has been subjected to the vertical DC erasing using the gaps,the servo signal, which has been yielded by reading the servo patternthus formed, has a unipolar pulse shape. In contrast, when a magneticpattern is transferred to the magnetic tape which has been subjected tothe parallel DC erasing, the servo signal, which has been yielded byreading the servo pattern thus formed, has a bipolar pulse shape.

Magnetic Recording and Reproducing Device

One aspect of the invention relates to a magnetic recording andreproducing device including the magnetic recording medium and amagnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of information on the magnetic recording medium or thereproducing of information recorded on the magnetic recording medium.Such a device is generally called a drive. The magnetic recording andreproducing device can be a sliding type magnetic recording andreproducing device. The magnetic head included in the magnetic recordingand reproducing device can be a recording head capable of performing therecording of information on the magnetic recording medium, and can alsobe a reproducing head capable of performing the reproducing ofinformation recorded on the magnetic recording medium. In addition, inthe aspect, the magnetic recording and reproducing device can includeboth of a recording head and a reproducing head as separate magneticheads. In another aspect, the magnetic head included in the magneticrecording and reproducing device can also have a configuration ofcomprising both of a recording element and a reproducing element in onemagnetic head. As the reproducing head, a magnetic head (MR head)including a magnetoresistive (MR) element capable of reading informationrecorded on the magnetic recording medium with excellent sensitivity asthe reproducing element is preferable. As the MR head, variouswell-known MR heads can be used. In addition, the magnetic head whichperforms the recording of information and/or the reproducing ofinformation may include a servo pattern reading element. Alternatively,as a head other than the magnetic head which performs the recording ofinformation and/or the reproducing of information, a magnetic head(servo head) comprising a servo pattern reading element may be includedin the magnetic recording and reproducing device.

In the magnetic recording and reproducing device, the recording ofinformation on the magnetic recording medium and the reproducing ofinformation recorded on the magnetic recording medium can be performedby bringing the surface of the magnetic layer of the magnetic recordingmedium into contact with the magnetic head and sliding. The magneticrecording and reproducing device may include the magnetic recordingmedium according to the aspect of the invention, and well-knowntechnologies can be applied for the other configurations.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and “%by mass”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

Example 1

A list of each layer forming composition is shown below.

List of Magnetic Layer Forming Composition

Magnetic Liquid

Ferromagnetic powder (see Table 1): 100.0 parts

Oleic acid: 2.0 parts

Vinyl chloride copolymer (MR-104 manufactured by Kaneka Corporation):10.0 parts

-   -   (Weight-average molecular weight: 55,000, active        hydrogen-containing group (hydroxy group): 0.33 meq/g, OSO₃K        group (potassium salt of sulfate group): 0.09 meq/g)

SO₃Na group-containing polyurethane resin: 4.0 parts

-   -   (Weight-average molecular weight: 70,000, active        hydrogen-containing group (hydroxy group): 4 to 6 mgKOH/g, SO₃Na        group (sodium salt of sulfonic acid group): 0.07 meq/g)

Polyalkyleneimine-based polymer (synthesis product obtained by methoddisclosed in paragraphs 0115 to 0123 of JP2016-051493A): 6.0 parts

Methyl ethyl ketone: 150.0 parts

Cyclohexanone: 150.0 parts

Abrasive Solution

α-alumina (Brunauer-Emmett-Teller (BET) specific surface area: 19 m²/g):6.0 parts SO₃Na group-containing polyurethane resin: 0.6 parts

-   -   (Weight-average molecular weight: 70,000, SO₃Na group: 0.1        meq/g) 2,3-Dihydroxynaphthalene: 0.6 parts

Cyclohexanone: 23.0 parts

Projection Forming Agent Liquid

Colloidal silica (average particle size: 120 nm): see Table 1

Methyl ethyl ketone: 8.0 parts

Other components

Stearic acid: 3.0 parts

Stearic acid amide: 0.3 parts

Butyl stearate: 6.0 parts

Methyl ethyl ketone: 110.0 parts

Cyclohexanone: 110.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by TosohCorporation): 3 parts

List of Non-Magnetic Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide (average particle size: 10nm, BET specific surface area: 75 m²/g): 100.0 parts

Carbon black (average particle size: 20 nm): 25.0 parts

SO₃Na group-containing polyurethane resin (weight-average molecularweight: 70,000, content of SO₃Na group: 0.2 meq/g): 18.0 parts

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

List of Back Coating Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide (average particle size: 0.15μm, BET specific surface area: 52 m²/g): 80.0 parts

Carbon black (average particle size: 20 nm): 20.0 parts

Vinyl chloride copolymer: 13.0 parts

Sulfonic acid salt group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Cyclohexanone: 155.0 parts

Methyl ethyl ketone: 155.0 parts

Stearic acid: 3.0 parts

Butyl stearate: 3.0 parts

Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

Preparation of Magnetic Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

A magnetic liquid was prepared by dispersing (beads-dispersing) variouscomponents of the magnetic liquid with a batch type vertical sand millfor 24 hours. As dispersion beads, zirconia beads having a bead diameterof 0.5 mm were used.

Regarding the abrasive solution, various components of the abrasivesolution were mixed with each other and put in a transverse beads milldispersing device together with zirconia beads having a bead diameter of0.3 mm, so as to perform the adjustment so that a value of beadvolume/(abrasive solution volume+bead volume) was 80%, the beads milldispersion process was performed for 120 minutes, the liquid after theprocess was extracted, and an ultrasonic dispersion filtering processwas performed by using a flow type ultrasonic dispersion filteringdevice. By doing so, the abrasive solution was prepared.

The prepared magnetic liquid, the abrasive solution, the projectionformation agent liquid, and the other components were introduced in adissolver stirrer, and stirred at a circumferential speed of 10 m/secfor 30 minutes. Then, a process at a flow rate of 7.5 kg/min wasperformed for 3 passes with a flow type ultrasonic dispersing device,and then, the mixture was filtered with a filter having a hole diameterof 1 μm, to prepare a magnetic layer forming composition.

Preparation of Non-Magnetic Layer Forming Composition

A non-magnetic layer forming composition was prepared by dispersingvarious components of the non-magnetic layer forming compositiondescribed above with a batch type vertical sand mill by using zirconiabeads having a bead diameter of 0.1 mm for 24 hours, and then performingfiltering with a filter having an average hole diameter of 0.5 μm.

Preparation of Back Coating Layer Forming Composition

Components except a lubricant (stearic acid and butyl stearate),polyisocyanate, and 200.0 parts of cyclohexanone among variouscomponents of the back coating layer forming composition were kneadedand diluted by an open kneader, and subjected to a dispersion process of12 passes, with a transverse beads mill dispersing device and zirconiabeads having a bead diameter of 1 mm, by setting a bead fillingpercentage as 80 volume %, a circumferential speed of rotor distal endas 10 m/sec, and a retention time for 1 pass as 2 minutes. After that,the remaining components were added and stirred with a dissolver, theobtained dispersion liquid was filtered with a filter having an averagehole diameter of 1 μm and a back coating layer forming composition wasprepared.

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition prepared as described abovewas applied onto a surface of a polyethylene naphthalate support havinga thickness of 5.0 so that the thickness after the drying becomes 400nm, and dried to form a non-magnetic layer. Then, the magnetic layerforming composition prepared as described above was applied onto asurface of the non-magnetic layer so that the thickness after the dryingbecomes 70 nm, to form a coating layer. A homeotropic alignment processof applying a magnetic field having strength of 0.3 T to the surface ofthe coating layer in a vertical direction while the coating layer of themagnetic layer forming composition is wet (not dried), and the coatinglayer was dried. After that, the back coating layer forming compositionprepared as described above was applied on the opposite surface of thesupport so that the thickness after drying becomes 0.4 μm, and dried. Bydoing so, a magnetic tape original roll was manufactured.

The calender process (surface smoothing treatment) was performed on themanufactured magnetic tape original roll with a calender configured ofonly a metal roll, at a speed of 100 m/min, linear pressure of 300 kg/cm(294 kN/m), and a surface temperature of a calender roll of 100° C., andheat treatment was performed in the environment of the atmospheretemperature shown in Table 1 for a period of time shown in Table 1.After the heat treatment, a magnetic tape having a width of ½ inches(0.0127 meters) was obtained by slitting the magnetic tape original rollwith a cutter. While causing this magnetic tape to run between a sendingroller and a winding roller (running speed: 120 m/min, tension: seeTable 1), blade polishing of the surface of the magnetic layer, the drywiping treatment, and the methyl ethyl ketone wiping treatment wereperformed in this order. Specifically, a sapphire blade, a dried wipingmaterial (TORAYSEE (registered trademark) manufactured by TorayIndustries, Inc.), and a wiping material permeated with methyl ethylketone (TORAYSEE (registered trademark) manufactured by TorayIndustries, Inc.) were disposed between the two rollers described above,the sapphire blade was pressed against the surface of the magnetic layerof the magnetic tape running between the two rollers for bladepolishing, the dry wiping treatment of the surface of the magnetic layerwas performed with the dried wiping material, and the methyl ethylketone wiping treatment of the surface of the magnetic layer wasperformed with the wiping material permeated with methyl ethyl ketone.By doing so, the blade polishing, the dry wiping treatment, and themethyl ethyl ketone wiping treatment were performed on the surface ofthe magnetic layer once.

By doing so, a magnetic tape of Example 1 was obtained.

Examples 2 to 8 and Comparative Examples 1 to 6

Magnetic tapes were manufactured by the same method as in Example 1,except that various conditions were changed as shown in Table 1.

Regarding the surface treatment of the surface of the magnetic layerafter the slitting, in Examples 2 to 4, 6 to 8 and Comparative Examples4 and 5, the blade polishing, the dry wiping treatment, and the methylethyl ketone wiping treatment were performed in the same manner as inExample 1.

In Example 5 and Comparative Example 6, the blade polishing, the drywiping treatment, and the methyl ethyl ketone wiping treatment wereperformed in the same manner as in Example 1, except that the tensionwas changed.

In Comparative Examples 1 and 3, the blade polishing and the dry wipingtreatment were performed in the same manner as in Example 1, and themethyl ethyl ketone wiping treatment was not performed.

In Comparative Example 2, the blade polishing and the dry wipingtreatment were repeatedly performed in the same manner as in Example 1three times, and methyl ethyl ketone wiping treatment was not performed.

In Table 1, “BaFe” is hexagonal barium ferrite powder having an averageparticle size (average plate diameter) of 21 nm.

In Table 1, “SrFe1” is the hexagonal strontium ferrite powder preparedby the following method.

1,707 g of SrCO₃, 687 g of H₃BO₃, 1,120 g of Fe₂O₃, 45 g of Al(OH)₃, 24g of BaCO₃, 13 g of CaCO₃, and 235 g of Nd₂O₃ were weighed and mixedwith a mixer to obtain a raw material mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1,390° C., a tap hole provided on the bottom ofthe platinum crucible was heated while stirring the melted liquid, andthe melted liquid was extracted in a rod shape at approximately 6 g/sec.The extracted liquid was rolled and rapidly cooled with a water-cooledtwin roller to manufacture an amorphous material.

280 g of the manufactured amorphous material was put into an electricfurnace and heated to 635° C. (crystallization temperature) at a rate oftemperature increase of 3.5° C./min, and held at the same temperaturefor 5 hours, to precipitate (crystallize) hexagonal strontium ferriteparticles.

Then, a crystalline material obtained above including the hexagonalstrontium ferrite particles was coarsely crushed with a mortar andsubjected to a dispersion process with a paint shaker for 3 hours, byadding 1,000 g of zirconia beads having a particle diameter of 1 mm and800 ml of acetic acid having a concentration of 1% in a glass bottle.After that, the obtained dispersion liquid was separated from the beadsand put into a stainless steel beaker. A dissolving process of the glasscomponent was performed by leaving the dispersion liquid at a liquidtemperature of 100° C. for 3 hours, the precipitation was performed witha centrifugal separator, decantation was repeated for washing, and theresultant material was dried in a heating furnace at a temperature inthe furnace of 110° C. for 6 hours, thereby obtaining hexagonalstrontium ferrite powder.

The hexagonal strontium ferrite powder obtained above had an averageparticle size of 18 nm, an activation volume of 902 nm³, an anisotropyconstant of 2.2×10⁵ J/m³, and a mass magnetization as of 49 A·m²/kg.

12 mg of sample powder was collected from the hexagonal strontiumferrite powder obtained above, element analysis of filtrate obtained bypartially dissolving the sample powder under the dissolving conditionsexemplified above was performed by the ICP analysis device, and thesurface portion content of neodymium atom was obtained.

Separately, 12 mg of sample powder was collected from the hexagonalstrontium ferrite powder obtained above, element analysis of filtrateobtained by totally dissolving the sample powder under the dissolvingconditions exemplified above was performed by the ICP analysis device,and the surface portion content of neodymium atom was obtained.

In the hexagonal strontium ferrite powder, the content (bulk content) ofneodymium atom with respect to 100 atom % of iron atom was 2.9 atom %,and the surface portion content of neodymium atom was 8.0 atom %. The“surface portion content/bulk content”, that is a ratio of the surfaceportion content to the bulk content, was 2.8. It was confirmed that theneodymium atom was unevenly distributed in the surface portion of theparticles.

The X-ray diffraction analysis of the powder obtained above wasperformed by scanning with a CuKα ray at a voltage of 45 kV andintensity of 40 mA and by measuring X-ray diffraction pattern under theconditions. By the X-ray diffraction analysis, it was confirmed that thepowder obtained above showed the crystal structure of hexagonal ferrite.The powder obtained above showed a crystal structure of magnetoplumbitetype (M type) hexagonal ferrite. In addition, a crystal phase detectedby the X-ray diffraction analysis was a magnetoplumbite type singlephase.

PANalytical X'Pert Pro diffractometer, PlXcel detector

Soller slit of incident beam and diffraction beam: 0.017 radians

Fixed angle of dispersion slit: 1/4 degrees

Mask: 10 mm

Scattering prevention slit: ¼ degrees

Measurement mode: continuous

Measurement time per 1 stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degrees

In Table 1, “SrFe2” is the hexagonal strontium ferrite powder preparedby the following method.

At first, 1,725 g of SrCO₃, 666 g of H₃BO₃, 1,332 g of Fe₂O₃, 52 g ofAl(OH)₃, 34 g of CaCO₃, and 141 g of BaCO₃ were weighed, and were thenmixed with a mixer to obtain a raw material mixture.

The obtained raw material mixture was dissolved in a platinum crucibleat a melting temperature of 1380° C., a tap hole provided on the bottomof the platinum crucible was heated while stirring the melted liquid,and the melted liquid was extracted in a rod shape at approximately 6g/sec. The extracted liquid was rolled and rapidly cooled with awater-cooled twin roller to manufacture an amorphous material.

Then, 280 g of the obtained amorphous material was placed in an electricfurnace, the temperature in the electric furnace was raised to 645° C.(crystallization temperature), and the amorphous material was stillstood in the electric furnace for 5 hours at the same temperature, toprecipitate (crystalize) hexagonal strontium ferrite particles.

Subsequently, the above-obtained crystal containing hexagonal strontiumferrite particles was roughly ground in a mortar, and the groundcrystals was put in a glass bottle, together with 1000 g of zirconiabeads having a particle diameter of 1 mm and 800 ml of acetic acidhaving a concentration of 1% and were subjected to a dispersiontreatment for 3 hours with a paint shaker. Thereafter, the obtaineddispersion was separated from the beads and put in a stainless beaker.The dispersion was stood still at a liquid temperature of 100° C. for 3hours to dissolve a glass component, and thereafter centrifuged in acentrifugal separator to precipitation and were repeatedly decanted towash the precipitated matter and the precipitated matter is dried in afurnace at an in-furnace temperature of 110° C. for 6 hours, to obtainhexagonal strontium ferrite powder.

The obtained hexagonal strontium ferrite powder had an average particlesize of 19 nm, an activation volume of 1102 nm³, an anisotropy constantKu of 2.0×10⁵ J/m³, and a mass magnetization σs of 50 A·m²/kg.

In Table 1, “ε-iron oxide” is the ε-iron oxide powder prepared by thefollowing method.

A solution was prepared by dissolving 8.3 g of iron(III) nitratenonahydrate, 1.3 g of gallium(III) nitrate octahydrate, 190 mg ofcobalt(II) nitrate hexahydrate, 150 mg of titanium(IV) sulfate, and 1.5g of polyvinylpyrrolidone (PVP) in 90 g of pure water. While stirringthe solution using a magnetic stirrer, 4.0 g of aqueous ammonia solutionhaving a concentration of 25% was then added to the solution in theatmosphere under a condition of an ambient temperature of 25° C. andstirred for subsequent 2 hours under the same ambient temperature of 25°C. A citric acid solution, which was obtained by dissolving 1 g ofcitric acid in 9 g of pure water, was added to the obtained solution,and the obtained mixture was then stirred for 1 hour. Powderprecipitated after the stirring was collected by centrifugal separation,washed with pure water, and dried in a furnace at an in-furnacetemperature of 80° C.

To the dried powder, 800 g of pure water was added to disperse thepowder in water again for preparing a dispersion. The obtaineddispersion was heated at a liquid temperature of 50° C., and 40 g ofaqueous ammonia solution having a concentration of 25% was addeddropwise thereto while stirring the dispersion. The dispersion wasstirred for 1 hour while maintaining the liquid temperature at 50° C.,and 14 mL of tetraethoxysilane (TEOS) was then added dropwise to thedispersion, and the obtained mixture was then stirred for 24 hours. Tothe obtained reaction solution, 50 g of ammonium sulfate was added, andprecipitated powder was then collected by centrifugal separation, washedwith pure water, and dried in a furnace at an in-furnace temperature of80° C., to obtain a ferromagnetic powder precursor.

The obtained ferromagnetic powder precursor was put in a furnace at anin-furnace temperature of 1000° C. in the atmosphere and heat-treatedfor 4 hours.

The heat-treated ferromagnetic powder precursor was introduced into a 4mol/L aqueous sodium hydroxide (NaOH) solution, and then stirred for 24hours while maintaining a liquid temperature at 70° C. to removeimpurity silicate compound from the ferromagnetic powder precursorsubjected to the heat treatment.

Thereafter, the ferromagnetic powder from which a silicate compound hasbeen removed was collected by a centrifugal separation and washed withpure water, to obtain ferromagnetic powder.

The composition of the obtained ferromagnetic powder was analyzed byinductively coupled plasma-optical emission spectrometry (ICP-OES) andwas found to be Ga, Co, and Ti substitution-type ε-iron oxide(ε-Ga_(0.28)Co_(0.05)Ti_(0.05)Fe_(1.62)O₃). Moreover, the obtainedferromagnetic powder was analyzed by X-ray diffraction analysis underthe same conditions as described for SrFe1 above, and it was confirmedfrom peaks in the X-ray diffraction pattern that the obtainedferromagnetic powder did not have crystal structures of α phase and γphase and had a single crystal structure of ε phase (ε-iron oxide typecrystal structure).

The obtained ε-iron oxide powder had an average particle size of 12 nm,an activation volume of 746 nm³, an anisotropy constant Ku of 1.2×10⁵J/m³, and a mass magnetization as of 16 A·m²/kg.

The activation volume and anisotropy constant Ku of each of thehexagonal strontium ferrite powder and the ε-iron oxide powder werevalues determined by the above-described method using a vibrating samplemagnetometer (manufactured by Toei Industry Co., Ltd.).

Moreover, the mass magnetization as is a value measured using avibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.)at a magnetic field strength of 15 kOe.

Evaluation of Magnetic Tape

(1) Center Line Average Surface Roughness Ra measured regarding Surfaceof Magnetic Layer (Magnetic Layer Surface Roughness Ra)

The measurement regarding a measurement area of 40 μm×40 μm in thesurface of the magnetic layer of the magnetic tape was performed with anatomic force microscope (AFM, Nanoscope 4 manufactured by VeecoInstruments, Inc.) in a tapping mode, and a center line average surfaceroughness Ra was acquired. RTESP-300 manufactured by BRUKER is used as aprobe, a scan speed (probe movement speed) was set as 40 μm/sec, and aresolution was set as 512 pixel×512 pixel.

(2) Spacing Difference (S_(after)−S_(before)) before and after MethylEthyl Ketone Cleaning

The spacing difference (S_(after) S_(before)) before and after themethyl ethyl ketone cleaning was obtained with a Tape Spacing Analyzer(TSA) (manufactured by Micro Physics, Inc.) by the following method.

Two test pieces having a length of 5 cm were cut out from each magnetictape of the examples and the comparative examples. Regarding one testpiece, the methyl ethyl ketone cleaning was not performed and thespacing (S_(before)) was obtained by the following method. Regarding theother test piece, the methyl ethyl ketone cleaning was performed by themethod described above, and the spacing (S_(after)) was obtained by thefollowing method.

In a state where a glass plate (glass plate (model no.: WG10530)manufactured by Thorlabs, Inc.) comprised in TSA is disposed on thesurface of the magnetic layer of the magnetic tape (specifically, thetest piece), a urethane hemisphere comprised in TSA as a pressing memberwas pressed against the surface of the back coating layer of themagnetic tape with pressure of 5.05×10⁴ N/m (0.5 atm). In this state, acertain region (150,000 to 200,000 μm²) of the surface of the magneticlayer of the magnetic tape was irradiated with white light from astroboscope comprised in the TSA through the glass plate, the obtainedreflected light was received with a charge-coupled device (CCD) throughan interference filter (filter selectively transmitting light at awavelength of 633 nm), thereby obtaining an interference fringe imagegenerated on ruggedness of this region.

This image was divided into 300,000 points, a distance (spacing) betweenthe surface of the glass plate of each point on the magnetic tape sideand the surface of the magnetic layer of the magnetic tape was obtained,this spacing is shown with a histogram, a mode S_(before) of thehistogram obtained regarding the test piece not subjected to the methylethyl ketone cleaning was subtracted from a mode S_(after) of thehistogram obtained regarding the test piece after the methyl ethylketone cleaning, and the difference (S_(after)−S_(before)) was obtained.

(3) Spacing Difference (S_(reference)−S_(before)) before and aftern-Hexane Cleaning (Reference Value)

One test piece having a length of 5 cm was further cut out from eachmagnetic tape of the examples and the comparative examples, the cleaningwas performed in the same manner as described above, except thatn-hexane was used instead of methyl ethyl ketone, and the spacing wasobtained after n-hexane cleaning in the same manner as described above.A difference (S_(reference)−S_(before)) between the spacingS_(reference) obtained here as a reference value, and the spacingS_(before) obtained from the test piece not subjected to the cleaningobtained in the section of (2) was obtained.

(4) Electromagnetic Conversion Characteristics (Signal-To-Noise-Ratio(SNR))

In an environment of an atmosphere temperature of 23° C. and relativehumidity of 50%, in each magnetic tape of the examples and thecomparative examples, a recording head (metal-in-gap (MIG) head, gaplength of 0.15 μm, track width of 1.0 μm, 1.8 T) and a reproducing head(giant magnetoresistive (GMR) head, element thickness of 15 nm, distancebetween shields of 0.1 μm, track width of 1.0 μm) were attached to aloop tester, and a signal having linear recording density of 325 kfciwas recorded. The unit kfci is a unit of linear recording density (notconvertible into SI unit). After that, the reproducing output wasmeasured and an SNR was obtained as a ratio of the reproducing outputand noise. The SNR was obtained as a relative value in a case where theSNR of Comparative Example 1 was 0 dB. In a case where the SNR obtainedhere is equal to or greater than 0 dB, it is possible to evaluate thatexcellent electromagnetic conversion characteristics are obtained.

(5) SNR decrease due to Temperature Change from Low Temperature to HighTemperature under High Humidity

After the evaluation of the electromagnetic conversion characteristicsin the section of (4), each magnetic tape of the examples and thecomparative examples was stored in a thermo box in which a temperaturewas 10° C. and relative humidity was 80%, for 3 hours. After that, themagnetic tape was extracted from the thermo box (in the outside air, atemperature was 23° C. and relative humidity was 50%), and put in athermo room in which a temperature was 32° C. and relative humidity was80% within 1 minute, the recording and reproducing were performed in thesame manner as in the section of (4) in the thermo room within 30minutes, and an SNR was obtained as a relative value, in a case wherethe SNR of Comparative Example 1 obtained in the section of (4) was 0dB. Regarding each magnetic tape of the examples and the comparativeexamples, a difference between the SNR obtained here and the SNRobtained in the section of (4) (“SNR obtained in this section”-“SNRobtained in the section of (4)”) was calculated and set as an SNRdecrease. In a case where the SNR decrease obtained here is within −1.0dB, it is possible to evaluate that a decrease in SNR due to atemperature change from a low temperature to a high temperature underhigh humidity is prevented.

The result is shown in Table 1 (Tables 1-1 and 1-2).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Ferromagnetic powder BaFe BaFe BaFe BaFe BaFe SrFe1SrFe2 ε-iron oxide Content of colloidal silica in 2.0 parts by 1.5 partsby 1.0 part by 2.0 parts by 2.0 parts by 2.0 parts by 2.0 parts by 2.0parts by projection forming agent liquid mass mass mass mass mass massmass mass Heat treatment Temperature 70° C. 70° C. 70° C. 80° C. 80° C.70° C. 70° C. 70° C. Time 36 hours 36 hours 36 hours 48 hours 48 hours36 hours 36 hours 36 hours Tension (N) 0.294 0.294 0.294 0.294 0.5880.294 0.294 0.294 Blade polishing and dry wiping 1 time 1 time 1 time 1time 1 time 1 time 1 time 1 time treatment Methyl ethyl ketone wipingtreatment 1 time 1 time 1 time 1 time 1 time 1 time 1 time 1 timeMagnetic layer surface roughness Ra 1.5 nm 1.3 nm 1.1 nm 1.5 nm 1.5 nm1.5 nm 1.5 nm 1.5 nm (Reference value) 2.0 2.0 2.0 2.0 1.5 2.0 2.0 2.0Spacing difference (S_(reference) − S_(before)) before and aftern-hexane cleaning (nm) Spacing difference (S_(after) − S_(before)) 12.010.0 11.0 7.0 3.0 12.0 12.0 12.0 before and after methyl ethyl ketonecleaning (nm) SNR 0 dB 0.8 dB 2.3 dB 0 dB 0 dB 1.6 dB 0.7 dB 2.4 dB SNRdecrease −0.4 dB −0.6 dB −0.9 dB −0.2 dB −0 dB −0.4 dB −0.4 dB −0.4 dB

TABLE 1-2 Comparative Comparative Comparative Comparative ComparativeComparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6BaFe BaFe BaFe BaFe BaFe BaFe Content of colloidal silica in 2.0 partsby mass 2.0 parts by mass 2.0 parts by mass 0.5 parts by mass 3.0 partsby mass 2.0 parts by mass projection forming agent liquid Heat treatmentTemperature 60° C. 60° C. 70° C. 70° C. 70° C. 80° C. Time 24 hours 24hours 36 hours 36 hours 36 hours 48 hours Tension (N) 0.294 0.294 0.2940.294 0.294 1.960 Blade polishing and dry wiping 1 time 3 times 1 time 1time 1 time 1 time treatment Methyl ethyl ketone wiping Not performedNot performed Not performed 1 time 1 time 1 time treatment Magneticlayer surface roughness 1.5 nm 1.5 nm 1.5 nm 0.9 nm 1.8 nm 1.5 nm Ra(Reference value) 2.0 2.0 2.0 2.0 2.0 0 Spacing difference(S_(reference) − S_(before)) before and after n-hexane cleaning (nm)Spacing difference 20.0 19.0 18.0 12.0 14.0 0 (S_(after) − S_(before))before and after methyl ethyl ketone cleaning (nm) SNR 0 dB 0.1 dB 0 dB3.9 dB −0.7 dB 0.1 dB SNR decrease −5.0 dB −2.0 dB −2.1 dB −3.0 dB −0.3dB −1.8 dB

As shown in Table 1, in the magnetic tapes of the examples, the magneticlayer surface roughness Ra is 1.0 nm to 1.6 nm, and the magnetic layerhaving a high surface smoothness is obtained. From the evaluation resultof the electromagnetic conversion characteristics (SNR), it is possibleto confirm that the magnetic tapes of the examples have excellentelectromagnetic conversion characteristics.

In addition, in the magnetic tapes of the examples, the surfacesmoothness of the magnetic layer is high and the spacing difference(S_(after)−S_(before)) before and after methyl ethyl ketone cleaning isgreater than 0 nm and equal to or smaller than 15.0 nm. As shown inTable 1, in the magnetic tapes of the examples, the SNR decrease issmaller than those in the magnetic tapes of the comparative examples,even in a case where the magnetic tape is exposed to a temperaturechange from a low temperature to a high temperature under high humidity.

In addition, as shown in Table 1, there is no correlation between thevalue of the spacing difference (S_(reference)−S_(before)) before andafter n-hexane cleaning and the value of the spacing difference(S_(after)−S_(before)) before and after methyl ethyl ketone cleaning.

One aspect of the invention is effective in a technical field of amagnetic recording medium for various data storage.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer including a ferromagneticpowder and a binding agent on the non-magnetic support, wherein a centerline average surface roughness Ra measured regarding a surface of themagnetic layer is 1.0 nm to 1.6 nm, and a differenceS_(after)−S_(before) between a spacing S_(after) measured by opticalinterferometry regarding the surface of the magnetic layer after methylethyl ketone cleaning and a spacing S_(before) measured by opticalinterferometry regarding the surface of the magnetic layer before methylethyl ketone cleaning is greater than 0 nm and equal to or smaller than15.0 nm.
 2. The magnetic recording medium according to claim 1, whereinthe difference S_(after)−S_(before) is 2.0 nm to 15.0 nm.
 3. Themagnetic recording medium according to claim 1, wherein the differenceS_(after)−S_(before) is 3.0 nm to 12.0 nm.
 4. The magnetic recordingmedium according to claim 1, further comprising: a non-magnetic layerincluding a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 5. The magnetic recordingmedium according to claim 1, further comprising: a back coating layerincluding a non-magnetic powder and a binding agent on a surface of thenon-magnetic support opposite to a surface provided with the magneticlayer.
 6. The magnetic recording medium according to claim 1, which is amagnetic tape.
 7. A magnetic recording and reproducing devicecomprising: a magnetic recording medium and a magnetic head, wherein themagnetic recording medium is a magnetic recording medium comprising: anon-magnetic support; and a magnetic layer including a ferromagneticpowder and a binding agent on the non-magnetic support, wherein a centerline average surface roughness Ra measured regarding a surface of themagnetic layer is 1.0 nm to 1.6 nm, and a differenceS_(after)−S_(before) between a spacing S_(after) measured by opticalinterferometry regarding the surface of the magnetic layer after methylethyl ketone cleaning and a spacing S_(before) measured by opticalinterferometry regarding the surface of the magnetic layer before methylethyl ketone cleaning is greater than 0 nm and equal to or smaller than15.0 nm.
 8. The magnetic recording and reproducing device according toclaim 7, wherein the difference S_(after)−S_(before) is 2.0 nm to 15.0nm.
 9. The magnetic recording and reproducing device according to claim7, wherein the difference S_(after)−S_(before) is 3.0 nm to 12.0 nm. 10.The magnetic recording and reproducing device according to claim 7,wherein the magnetic recording medium further comprises a non-magneticlayer including a non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.
 11. The magnetic recordingand reproducing device according to claim 7, wherein the magneticrecording medium further comprises a back coating layer including anon-magnetic powder and a binding agent on a surface of the non-magneticsupport opposite to a surface provided with the magnetic layer.
 12. Themagnetic recording and reproducing device according to claim 7, whereinthe magnetic recording medium is a magnetic tape.